Low side lobe pillbox antenna employing open-ended baffles



Oct. 12, 1965 J. s. AJIOKA LOW SIDE LOBE PILLBOX ANTENNA EMPLOYING OPEN-ENDED BAFFLES 3 Sheets-Sheet 1 Filed Feb. 14, 1963 LAYER RAY FIG. 2

TYPICAL RAY INVENTOR. JAMES S. AJIOKA Oct. 12, 1965 J. s. AJIOKA 3,212,095

LOW SIDE LOBE PILLBOX ANTENNA EMPLOYING OPEN-ENDED BAFFLES Filed Feb. 14, 1963 3 Sheets-Sheet 2 FIG. 7

INVENTOR.

6 JAMES S. AJ/OKA Oct. 12, 1965 J. 5, AJIOKA 3,212,095

LOW SIDE LOBE PILLBOX ANTENNA EMPLOYING OPEN-ENDED BAFFLES Filed Feb. 14, 1963 3 Sheets-Sheet 5 FIG. 5

INVENTOR. J4 MES S. AJ/OKA United States Patent 3,212,095 LOW SIDE LUBE PILLBOX ANTENNA EMPLOY- llNG OPEN=ENDED BAFFLES James S. Ajioka, Fullerton, Calif., assignor to the United States of America as represented by the Secretary of the Navy Filed Feb. 14, 1963, Ser. No. 258,973 5 Claims. ((21. 343780) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to an antenna and more particularly; to a pillbox antenna having improved side lobe and back lobe suppression characteristics and specifically; to a pillbox antenna having improved side lobe and back lobe response and so constructed that the feed and radiating apertures do not obstruct one another.

Patent No. 2,638,546 to Chu et al. describes a pillbox antenna comprising a short section of parabolic cylinder axially terminated by flat conductive plates and having another conductive plate positioned intermediate the conductive terminating plates. The feed, comprising the open end of a waveguide transmission line, is positioned at the focus of the parabolic reflector and is terminated between the intermediate plate and one of the plates terminating the reflector.

In such a system the feed probe does not obstruct the aperture and in addition energy is not reflected back into the feed in a substantial amount. Aperture blockage reduces gain and increases side lobes and reflection back to the feed makes broadbanding or impedance matching over a broad frequency band impossible Without the incorporation of a compensating device, which causes further degeneration of the pattern.

However, in the prior art the side lobe and back lobe and overall antenna pattern was still not suflicient for certain applications, for example, for air-tralfic identification and control systems where a continuing need exists for interrogation antennas with extremely low side lobes to combat the problem of spurious responses. In addition, an antenna development for IFF carries the added requirement that the IFF antenna be suitable for use with radar systems currently in use.

An object of the present invention is to provide an improved pillbox antenna having an aperture free from obstruction and having extremely low side lobes.

A further object of the present invention is to provide a pillbox antenna which is lightweight and may be used with existing radar systems as an IFF antenna.

An additional object of the present invention is to provide a pillbox antenna having improved azimuth patterns, elevation patterns and complete matching feasibility.

Another object of the present invention is to provide an improved pillbox antenna which is extremely lightweight and which may be used with existing radar systems as an IFF interrogation antenna wherein the possibility of spu rious responses due to side lobes has been drastically reduced.

A further object of the present invention is to provide an improved feed for an antenna system.

Various other objects and advantages will appear from the following description of one of the embodiments of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

These and other objects will be more apparent upon consideration of the following description, together with the accompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of the invention;

FIG. 2 is a plan view, partly in cross section, of the embodiment of FIG. 1;

FIG. 3 is a side elevation along the center line of FIG. 2;

FIG. 4 is a perspective view of another embodiment of the invention;

FIG. 5 is a bottom view, the embodiment of FIG. 4;

FIG. 6 is a plan view of the embodiment of FIG. 4; and

FIG. 7 is a side elevation of the antenna of FIG. 4;

FIG. 8 is -a perspective view, partly in section, of another embodiment of the invention.

With reference to the embodiment of FIG. 1, a conducting parabolic reflector, comprising a short section of parabolic cylinder 100, is closed at its ends by conducting top and bottom plates 101 and 102 respectively. An intermediate conducting plate 103 is provided having a parabolic edge 104 conforming to the curvature of parabolic reflector 100. This intermediate conducting plate 103 is fixed midway between the terminating plates 101 and 102, forming a septum, with a uniform separation being maintained between the parabolic edge 104 and the parabolic reflector 100. At approximately the focus of the parabolic reflector the three plates, i.e., plates 101, 102 and 103 are terminated. Plate 101 is flared upwardly at an angle of approximately 30 with respect to plate 103 approximately 1 from the vertex of the parabolic reflector 100, where f is the operating frequency of the IFF system.

The region enclosed between upper plate 101 and the intermediate plate 103 forms the aperture of the pillbox antenna. The intermediate plate 103 effectively separates the region encompassed by the parabolic surface 100 into two separate areas or layers with the feed in the lower or bottom layer between plate 102 and 103 and the aperture lying between plates 101 and 103.

A rectangular feedbox 105 is positioned in the lower layer between intermediate plate 103 and lower plate 102 substantially in the focus of the parabolic reflector. Positioned in the feedbox is a feed probe 106 to which the electromagnetic energy to be radiated and received is coupled. It is to be understood, of course, that the horn, through the property of reciprocity, will receive energy through the aperture as well as radiating energy through feed probe 106.

Spacers as at 107 are utilized to position the intermediate plate 103 accurately between upper plate 101 and lower plate 102. These may be formed of any suitable insulating material such as Micarta, Fiberglas or Styrofoam or tuned choke metallic separators. The upper and lower plates will be fabricated of a lightweight material such as riveted aluminum sheet metal while the center plate may be made of an aluminum honeycomb laminate which provides stiifness with minimum weight and permits adequate support by the few insulated spacers.

In addition, chokes or baflles 108 and 109 are positioned at the outer edge of upper plate 101 and intermediate plate 103 respectively. The purpose of these baflles or chokes will be explained in detail in the operation of the antennas.

An electromagnetic energy absorbent material 110 is provided in the lower layer adjacent to feedbox 105 on both sides thereof and extends to the outer edges of the pillbox antenna. Only one-half of this is shown in FIG. 2, however, it is to be understood the pillbox is symmetrical about the center line as shown.

The outer edges of the pillbox antenna terminate in sheet metal ends as at 111. Again, only one of the ends is shown.

The antenna may be also formed as shown in FIG. 4 wherein the upper and lower layers are formed of conducting rods. In this particular embodiment the bottom layer 200 is comprised of individual rods which radiate from a feedbox 201 within which the feed probe, not shown, is located. The rods terminate at the parabolic reflector portion 202 which in fact is formed by a continuation of the rods around to a top layer 203.

The top portion 203 is formed of the conducting rods also and in this embodiment the upper plate rods 203 are shown to extend beyond the bottom rods 200, however, this is done merely to provide a backing surface upon which the baflles, not shown in FIGS. 5 and 6, are mounted. However, these baflles are mounted as shown in FIG. 4.

A center plate 204 is provided which effectively divides the parabolic reflector region 202 into two separate chambers, an upper aperture chamber and a lower fee-d chamher.

The bafiles 205 and 206 are shown schematically in the end view of FIG. 7. Baflle 205 is mounted on the upper or top rods 203 and the other baffle 206 mounted on the center plate 204.

As shown, the rods on the top of the pillbox are parallel, and the current flows along the rods in the upper or aperture layer of the pillbox. The rods on the bottom of the pillbox radiate from the focus of the parabola and the current flow is from the feedbox along the rods in the lower or feed layer of the pillbox to the top rods.

The antenna in the embodiment of FIGS. 4 through 7 was built with the rods positioned along the ideal lines of current flow for a cylindrical TEM mode. The advantage in this construction over that of the solid sheet construction of FIGS. 1 through 3 lies in the fact that the currents are forced to flow along the rods. The advantage shows up in the reduced effect of the reflection at the bend. With rod construction, the bend reflection does not harm the aperture distribution and no absorbing material is necessary.

Without absorbing material, in the solid sheet construction of FIGS. 1 through 3, the reflection is re-reflected at the front edge of the lower layer of the pillbox and is transmitted around the bend and out the aperture with a circular wave front. This situation causes amplitude and phase errors in the aperture. With the absorbing material, i.e., Darkflex in the present embodiment, the reflected wave from the bend is absorbed and does not come out the aperture to disturb the aperture distribution.

In the case of the rod construction, as shown in FIGS. 4 through 7, the currents are forced to travel along the rods and the reflection from the bend comes back to the feedbox and never gets into the aperture. Since this reflection is small, it may easily be matched at the feed.

In the embodiment of FIG. 8 the folded pillbox is used as a line source feed to a linear array. Two different types of arrays are shown in FIG. 8, i.e., one array using polyrods and the other using slotted waveguides. However, it is to be understood that only one or the other of the above would be used at any one time in actual operation.

In this embodiment the pillbox comprises a short section of parabolic cylinder 300 terminated by top and bottom conducting plates 301 and 302 respectively. An intermediate conducting plate 303 is fixed midway between the terminating plates 301 and 302 forming a septum, with a uniform spacing between the parabolic edge of 303 and the reflector 300.

The three plates, 301, 302 and 303 are terminated at approximately the focus of the parabolic reflector 300. A feedbox 305 is positioned in the lower layer between plates 302 and 303 substantially in the focus of the reflector. A feed probe, not shown, is positioned in the feed box for providing excitation.

An absorbing material 306 such as Darkflex i shown as being positioned on one side of feed box 305 for absorbing electromagnetic energy when using an antenna array that introduces reflections. This would occur when using the slotted waveguides as an array.

one wave length for a flare angle of about 60.

An antenna array such as 307 or 308 may be used which provide low side lobe patterns with a constant phase and proper amplitude. Array 307 is formed of polyrods, i.e., rods constructed of Polystyrene while array 308 is constructed of slotted waveguide.

Operation In the operation of the antenna, the energy starting from the probe excited feedbox of either of the embodiments originates at the feedbox as at 105, travels back toward the parabolic reflecting surface in FIG. 2 and 202 of FIG. 5, turns at the bend and proceeds out the aperture into free space. The bend in the top surface in both instances, together with the chokes and baffles shape the elevation pattern of the antenna and also reduce the back radiation to an extremely low value. In operation, a dielectric weatherizing cover completely closes the aperture of the antenna, however, the antenna may operate with or without such a cover and such is not essential.

In the design of the present pillbox antenna an attempt was made to obtain as nearly as possible a Gaussian aperture distribution with a taper of 30 db to the edges. If the antenna aperture is large, a. steep Gaussian distribution would theoretically give very low side lobes; no side lobes with an infinite aperture. Of course, this low sidelobe would be obtained only by reducing gain and increasing the beamwidth, but in this development low side lobes are more important than gain.

It is known that the aperture amplitude distribution is due primarily to the shape of the primary feed pattern. The primary feed in all embodiments of the present invention is a rectangular resonating box excited by a probe. In the pillbox antenna, the feedbox is fastened between the bottom and the center plate and looks into a parallel plate region.

The size of the feedbox is chosen to give a secondary aperture amplitude distribution which closely approximated the Gaussian distribution.

When operating, the center plate of both embodiments is adjusted precisely to reduce reflection at the bend, however, even with a precise adjustment there is some small reflection. This reflection is re-reflected from the front edge of the bottom layer, i.e., feed layer of the pillbox and comes around the bend to the aperture with a circular wave front. This reflection added to the plain wave front of the initially transmitted, around the bend, portion of energy causes phase and amplitude error and raises the side lobes above 30 db. This rereflection is only incident to the embodiment of FIGS. 1-3 in that the rod type construction of FIGS. 4 through eliminates the re-refiected ray. The re-reflection can be essentially eliminated by microwave absorbing materials flanking the feedbox along the front edge of the feed layer as shown in FIG. 2. Since the re-reflection is very small there is no measurable loss in gain due to the addition of the absorbing material. The only effect on the antenna patterns is to reduce the side lobes and make the first null very distinct. However, the main beam is essentially unchanged.

Various kinds of absorbing loads may be used nearly all of them with some degree of success. The absorbing material does not have to be extremely good in the terms of quality. The commercially available Darkflex is used with success and in addition Styrofoam painted with aquadag is also successful. In addition a resistor arrangement may be used.

In order to shape the elevation pattern and reduce back lobes, baflles as shown in the two embodiments are utilized. In the present invention the aperture of the pillbox without chokes and baffles is essentially the same as the aperture of a horn, and it is known that horns with wide flare angle and short sides have considerable back radiation. For low wind loading, the length of the flare in the vertical, i.e., E-plane should be less than One reason for poor E-plane patterns of the horn is that there is no amplitude taper over the aperture, that is, the field is constant along E lines. To make matters worse in the present case, the phase front is circular due to the wide flare angle. If it were possible to taper the amplitude distribution and to straighten the phase front, good E-plane patterns might be obtained. This effect is approximately achieved by the use of chokes and baffles.

Amplitude taper is achieved by the use of bafiles. The uniform amplitude distribution of the simple horn has now been changed to a step-taper distribution. The amount of amplitude taper is determined by the size of the opening to the waveguide region. The baffles are held in place by vertical sheet-metal webs oriented in the E-plane between the sides of the flare and the baffles. The spaces between the baffles and the sides of the flare are Waveguide regions, the webs forming the narrow sides of the waveguides and the narrow ends of said battle waveguide sections being open to receive energy from the respective horn regions. The spacing between the webs determines the phase velocity in this region; if the webs are properly spaced, the phase velocity is increased to give a phase distribution that enhances the elevation pattern and reduces the back radiation.

In that one application of the antenna of the present invention is in the identification of aircraft, it is desirable to have a cosecant-squared power pattern in the vertical plane. An approximation to the cosecant-squared pattern is obtained in the present invention by adjusting the phase and amplitude distribution over the vertical aperture. It is known that asymmetrical phase distribution is necessary for a cosecant-squared pattern and for this reason non-symmetry was added in the vertical plane by bending the top surface of both embodiments of the present antenna 30 up at a distance from the vertex of the parabola. By means of the bend and the adjustment of the baflles and web, a cosecant-squared pattern is nearly achieved.

The antenna built with rods along the ideal lines of current flow as illustrated in FIGS. 4 through 7 utilizes a solid sheet for the intermediate conducting plate. However, it is possible to make the center conductive plate of a mesh such as that which would be formed by superposing the top and bottom rod structures.

In the operation of the embodiment of FIG. 8 the basic low side lobe folded pillbox design is used as a line source feed for a linear array of radiating elements. The radiation pattern of the linear array in the plane of the pillbox is determined by the pillbox design, i.e., low side in that plane while the pattern in a plane normal to the pillbox is determined by the directive pattern of the end-fire elements.

The combination of a low side lobe folded pillbox feeding an array of endfire elements results in a low silhouette, low wind-drag antenna with directivity in the vertical plane and a low side lobe, highly directive pattern in the azimuth, i.e., plane of the pillbox, plane.

Although the embodiment of FIG. 8 shows waveguide slot antennas and polyrod antennas as radiating elements of the linear array fed by the pillbox, any element such as a dipole may be used in general.

Through the use of a dual primary feed for the pillbox, monopulse operation can be achieved in the conventional manner for all of the antenna designs set forth in FIGS. 1-8.

In addition, through the use of the embodiment of FIG. 8 no complicated feed network is needed.

The antennas set forth are readily adaptable for use as a direct replacement or as a glue-on addition to existing radar antennas. This is due to the fact that the antennas are extremely lightweight and of a small size. Further, due to the fact that the antennas have side lobes well below minus 30 db the problem of spurious responses in connection with air-traflic identification and control system has been elimintaed to a considerable extent. Further, through the use of the materials such as sheet aluminum or lightweight aluminum honeycomb and the riveted construction of the pillbox antenna the cost thereof is extremely low.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have herein been described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A pillbox antenna comprising:

a parabolic reflecting cylinder axially terminated by substantially parallel flat conductive plates;

an intermediate substantially flat conductive plate centrally disposed between said substantially parallel flat conductive plates;

said intermediate plate having a substantially parabolic edge positioned toward said cylinder and conforming to the curvature of said parabolic reflector;

said parabolic edge being uniformly separated from said parabolic reflector;

said first mentioned substantially parallel flat conductive plates and said intermediate flat conductive plates having outer edges spaced from said parabolic reflector; one of said first mentioned substantially parallel flat conductive plates being flared outwardly;

electromagnetic feed means operatively positioned between the other of said substantially parallel flat con ductive plates and said intermediate flat conductive plates for supplying energy to said pillbox antenna; and

baflle means operatively positioned at the outer edge of said intermediate flat conductive plate and said outwardly flared plate of said substantially parallel flat conductive plates.

2. A pillbox antenna comprising;

a parabolic reflecting cylinder;

flat conductive plates axially terminating said parabolic reflecting cylinder; an intermediate conductive plate centrally disposed between said terminating plates and lying in a plane substantially parallel to the planes of said flat conductive plates axially terminating said parabolic reflecting cylinder, said intermediate plate having a substantially parabolic edge conforming to the curvature of said parabolic reflecting cylinder, said parabolic edge being uniformly separated from said parabolic cylinder; one of said flat conductive plates axially terminating said parabolic reflecting cylinder being outwardly flzlired with respect to said intermediate conductive p ate;

and baflle means operatively positioned at the outer edges of said outwardly flared flat conductive plate and said intermediate conductive plate for shaping the elevation pattern and reducing the back lobe from the antenna.

3. A pillbox antenna comprising;

a parabolic reflecting cylinder;

parallel conductive means axially terminating said parabolic reflecting cylinder;

intermediate conductive means centrally disposed between said parallel conductive means axially terminating said reflecting cylinder;

said intermediate means having a substantially parabolic edge conforming to the curvature of said parabolic cylinder, said parabolic edge being uniformly separated from said parabolic reflector;

one of said parallel conductive means axially terminat,

ing said parabolic reflecting cylinder being outwardly flared with respect to said intermediate conductive means;

electromagnetic feed means operatively positioned between the other of said parallel conductive means axially terminating said parabolic reflecting cylinder and said intermediate conductive means; and

baflle means operatively positioned at the outer edges of said outwardly flared conductive means and said intermediate conductive means for determining the elevation pattern of the radiated beam and reducing back lobes from the pillbox antenna.

4. An antenna as set forth in claim 3 wherein;

said parallel conductive means axially terminating said parabolic reflecting cylinder comprise conductive rods.

5. A pillbox antenna comprising;

a parabolic reflecting cylinder;

substantially parallel flat conductive plates axially terminating said reflecting cylinder;

an intermediate conductive plate centrally disposed between said plate axially terminating said reflecting cylinder and lying in a plane substantially parallel to said first mentioned conductive plates; said intermediate plate having a substantially parabolic edge conforming to the curvature of said parabolic cylinder, said parabolic edge being uniformly separated from said parabolic reflector thereby forming substantially two reflective portions;

one of said parallel conductive plates axially terminating said parabolic reflecting cylinder being outwardly flared at the outer edge thereof with respect to said intermediate conductive plate;

electromagnetic energy feed means operatively positioned between the other of said parallel conductive plates axially terminating said parabolic reflecting cylinder and said intermediate conductive plate;

electnomagnetic energy absorbent means positioned adjacent said electromagnetic energy feed means;

said electromagnetic energy absorbing means being operative to reduce reflected energy and thereby improve the side lobe pattern of the pillbox antenna;

and baffle means extending outwardly from the outer edges of said outwardly flared plate axially terminating said parabolic reflecting cylinder and said intermediate conductive plate for improving the elevation pattern of the antenna and reducing the back lobe thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,415,089 2/47 Feldman 343-776 2,469,419 5/49 Tawney 343-786 2,638,546 9/54 Chu 343-780 2,690,508 9/54 Cutler 343-780 2,751,586 6/56 Riblet 343-776 X 2,801,413 7/57 Beck 343-780 FOREIGN PATENTS 604,700 9/ Canada. 611,365 10/43 Great Britain.

OTHER REFERENCES Slayton, article, electronics, July 1955, Design Micro- 30 wave Horns, pages -151 relied upon.

HERMAN KARL SAALBACH, Primary Examiner. 

1. A PILLBOX ANTENNA COMPRISING: A PARABOLIC REFLECTING CYLINDER AXIALLY TERMINATED BY SUBSTANTIALLY PARALLEL FLAT CONDUCTIVE PLATES; AN INTERMEDIATE SUBSTANTIALLY FLAT CONDUCTIVE PLATE CENTRALLY DISPOSED BETWEEN SAID SUBSTANTIALLY PARALLEL FLAT CONDUCTIVE PLATES; SAID INTERMEDIATE PLATE HAVING A SUBSTANTIALLY PARABOLIC EDGE POSITIONED TOWARD SAID CYLINDER AND CONFORMING TO THE CURVATURE OF SAID PARABOLIC REFLECTOR; SAID PARABOLIC EDGE BEING UNIFORMLY SEPARATED FROM SAID PARABOLIC REFLECTOR; SAID FIRST MENTIONED SUBSTANTIALLY PARALLEL FLAT CONDUCTIVE PLATES AND SAID INTERMEDIATE FLAT CONDUCTIVE PLATES HAVING OUTER EDGES SPACED FROM SAID PARABOLIC REFLECTOR; ONE OF SAID FIRST MENTIONED SUBSTANTIALLY PARALLEL FLAT CONDUCTIVE PLATES BEING FLARED OUTWARDLY; ELECTROMAGNETIC FEED MEANS OPERATIVELY POSITIONED BETWEEN THE OTHER OF SAID SUBSTANTIALLY PARALLEL FLAT CONDUCTIVE PLATES AND SAID INTERMEDIATE FLAT CONDUCTIVE PLATES FOR SUPPLYING ENERGY TO SAID PILLBOX ANTENNA; AND BAFFLE MEANS OPERATIVELY POSITIONED AT THE OUTER EDGE OF SAID INTERMEDIATE FLAT CONDUCTIVE PLATE AND SAID OUTWARDLY FLARED PLATE OF SAID SUBSTANTIALLY PARALLEL FLAT CONDUCTIVE PLATES. 